Module 1  

Lesson 1.8  Isomers of Alkenes and Alkynes



Key Concepts


You will recall from your study of alkanes that a structural isomer is a compound with the same molecular formula but a different structure.  Alkenes and alkynes can form structural isomers, also.  For example, if the multiple bond changes position, an isomer is produced. 



Fig. 1  pent-2-ene


Fig. 2  pent-1-ene


Similarly, an isomer is also produced if a straight chain hydrocarbon is changed into a branched hydrocarbon or a cyclic hydrocarbon.  All the isomers of a molecule have the same number of carbons and the same number of hydrogens.  However, the atoms are arranged differently.  Refer to page 377 in your textbook.



Fig. 3  Hex-1-ene, C6H12


Fig. 4  Cyclohexane, C6H12


Hex-1-ene and cyclohexane both have the same molecular formulae (same number of carbon atoms and hydrogen atoms).    Therefore,  they are isomers of one another.



Fig. 5  Hex-1-yne, C6H10


Fig. 6  Cyclohexene, C6H10



Study the two diagrams above.  Cyclohexene and hex-1-yne have the same molecular formulae (same number of carbon atoms and hydrogen atoms).  Therefore, they are isomers of one another.

Learning Tip

Alkenes and cycloalkanes that have the same number of carbons and hydrogens are isomers of one another. Similarly, alkynes and cycloalkenes that have the same number of carbons and hydrogens are isomers of one another.


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Check Your Understanding


Go to your textbook and complete  Section 9.3 Question 2 and 5 on page 380. Check your answers by clicking on the link below.



Page 380 Section 9.3 Question 2
There are more possible isomers for an alkene because the position of its double bond can vary.

Page 380 Section 9.3 Question 5